Fleet monitoring: sysstat 28d + perfsnap/PSI/iotop timers + on-demand toolbox container (#826)

- monitoring/node-install.sh: idempotent per-node installer (sar HISTORY=28,
  three oneshot timers, per-node conf with qemu.slice/lxc cgroup globs)
- monitoring/collectors + systemd units: iotop-snap 15min, perfsnap 60s,
  psi-textfile 15s (now sources /etc/knelperf/perfsnap.conf; multi-glob)
- monitoring/toolbox: pct LXC (vmid 950, privileged, apparmor unconfined,
  stopped at rest) + knelperf-toolbox wrapper + Dockerfile variant
- monitoring/deploy-fleet.sh: workstation-side rollout driver
- piloted on pfv-tsys1: per-VM PSI gauges verified live

Redmine: https://projects.knownelement.com/issues/826

monitoring/collectors/knelperf-iotop-snap.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# KNELPerf iotop snapshot — periodic batch samples of active per-process I/O.
+# Complements sar (system-wide device stats) with the WHO behind the I/O.
+# Env (or /etc/knelperf/perfsnap.conf):
+#   KNELPERF_IOTOP_LOGDIR   default /var/log/knelperf-iotop
+# Output: daily file iotop-YYYYMMDD.log; per tick one header line then
+#   3 batch iterations x 2s of processes with outstanding I/O (-botqq -k).
+# Docs: https://community.turnsys.com/t/328
+set -u
+CONF=/etc/knelperf/perfsnap.conf
+[ -r "$CONF" ] && . "$CONF"
+LOGDIR=${KNELPERF_IOTOP_LOGDIR:-/var/log/knelperf-iotop}
+mkdir -p "$LOGDIR"
+F="$LOGDIR/iotop-$(date +%Y%m%d).log"
+{
+  echo "### $(date -Iseconds)"
+  iotop -botqq -k -n 3 -d 2 2>/dev/null
+} >> "$F"
+# retention: 14 days; rotate the live file at 25MB
+find "$LOGDIR" -name 'iotop-*.log*' -mtime +14 -delete 2>/dev/null
+[ "$(wc -c <"$F" 2>/dev/null || echo 0)" -gt 26214400 ] && mv "$F" "$F.1"
+exit 0